The present paper shows the results of the study of the protection against ultraviolet radiation provided by two optical brightening agents, with a different chemical structure, when applied on cotton fabrics. One of the products is a derivative of the stilbene disulphonic acid and the other a derivative of the distyryl biphenyl. The influence of the chemical structure and the concentration of both products have been evaluated. To this aim the ultraviolet absorption spectra of the products have been determined as well as the diffuse transmittance spectra of the fabrics treated with different concentrations of the products. The UPF (Ultraviolet Protection Factor) of the untreated and treated fabrics has been calculated, establishing the effectiveness of each product regarding their action in the UVA and UVB zones of the electromagnetic spectrum.